Orbital Interactions in Hydrogen Bonds: A Perspective From the Chemical Bond Overlap Model

Hydrogen bonds are essential chemical interactions that occur in various systems, playing a critical role in determining molecular structures, dynamics, and reactivity. While quantum chemical methods such as Quantum Theory of Atoms in Molecules (QTAIM) and Natural Bond Orbital (NBO) analyses have traditionally been used to explore these interactions, this work introduces the Chemical Bond Overlap (OP) Model and its topological (TOP) descriptors as a complementary approach for analyzing orbital overlap contributions in hydrogen bonds. The study reports a systematic investigation of a series of hydrogen-bonded systems (a total of 25 systems), demonstrating how electron-donating and electron-withdrawing substituents influence bond characteristics. The results reveal that OP/TOP effectively captures the effects of electronic perturbations, offering insights into the $n$(X) $\to$ ${\sigma }^{\ast }({\text{X}}^{\prime }-\text{H})$ interactions and serving as a complementary approach to QTAIM, NBO, and local vibrational modes theory (LVM). Notably, for nonconventional ${\left({\text{CH}}_3\right)}_3\text{N}\text{⋯}\text{H}\text{⋯}{\text{CX}}_3$ hydrogen bonds ($\text{X}=\text{F},\text{Cl}$), the OP/TOP model, consistent with LVM, correctly captures the expected increase in interaction strength for $\text{X}=\text{Cl}$. This agrees with the higher electrophilicity of the hydrogen in ${\text{HCCl}}_3$, as indicated by its lower pKa and weaker CH bond dissociation energy. Additionally, the inclusion of electron-donating groups significantly enhances lone pair $\to$ antibonding orbital interactions, increasing NBO ${\sigma }^{\ast }({\text{X}}^{\prime }-\text{H})$ occupancy and electron density at the hydrogen bond critical point (BCP), as reflected by a decrease in ${\text{H}}_{\text{BCP}}$ and an increase in ${\rho }_{\text{BCP}}$. This behavior consistently indicates hydrogen bond strengthening across QTAIM, NBO, and OP/TOP descriptors. Calculations were performed using the $\omega$B97X-D/def2-TZVP level of theory. The findings establish OP/TOP as a powerful tool for computational chemistry, particularly in the study of weak intermolecular interactions and molecular design.